Chapters 43 and 44: Emission Control Systems, and Diagnosis and Repair
40% of a vehicle's emissions
Emissions from fuel evaporation and crankcase vapors
60% of a vehicle's emissions
Engine exhaust
Higher than normal NoX readings
Excessively high compression ratio, carbon deposits in the combustion chamber, low coolant, blocked cooling system, stuck thermostat or EGR system problems
Carbon Monoxide (CO)
Extremely toxic, colorless and odorless gas that is caused by rich air-fuel mixtures; Indicates incomplete burning of fuel or rich mixtures
Diverter valve
Keeps air from entering the system during deceleration to prevent backfire
Air check valve
Keeps exhaust gases from entering the air injection system
Rollover valve
Keeps liquid fuel from entering the vent line if the vehicle rolls over
Higher than normal HC's
Leaking injector, faulty pressure regulator or high fuel pressure; Blow-by, worn rings or blown head gasket; Bad PCV, catalytic converter or evaporative control system; Fouled spark plug, defective coil or open spark plug wire
Engine designs that can minimize emissions:
Lower compression ratios Small combustion chamber surface volume Reduced quench areas Decreased valve overlap Hardened valves and seats
Fuel tank pressure sensor
Monitors fuel tank pressure, it measures vapor pressure (DOES NOT measure fuel pressure)
Oxygen (O2)
Needed for the catalytic converter to burn HC and CO emissions, indicator of a vehicle's air-fuel mixture; High readings indicate a lean mixture
Ported vacuum
Not present at idle with the throttle plate closed
Purge valve
Operated electrically or by vacuum, it controls the flow of fuel vapors; generally allows flow when the engine is operating above idle and at operating temperature
Emission control systems
Positive crankcase ventilation (PCV) Evaporative emissions control (EVAP) Exhaust gas recirculation (EGR) Air injection/ Pulse air injection Catalytic converter Computer control
Evaporative emissions control system (EVAP)
Prevents evaporating fuel vapors from entering the atmosphere; Fuel vapors are stored in a charcoal canister until they can be burned in the engine
Non-vented fuel cap
Prevents fuel vapors from entering the atmosphere, the pressure and vacuum relief valve will vent if fuel tank pressure or vacuum levels get too high
CO readings
Related to the air-fuel ratio: A high CO reading would indicate an over-rich mixture A low CO reading would indicate a lean air-fuel mixture
PCV valve testing
Remove the valve from the grommet and shake it, the valve should rattle freely; Start the engine and plug the valve with your finger, you should feel vacuum
Particulates
Solid particles of carbon soot and fuel additives, a serious problem with diesel engines
Charcoal canister
Stores fuel vapors when the engine is not running
Hydrocarbons (HC's)
Unburned fuel and all other petroleum-based products, produced by incomplete combustion or fuel evaporation
Enhanced EVAP system
Uses athe control module, solenoids and a pressure sensor to monitor the condition of the fuel system
Positive crankcase ventilation (PCV)
Uses engine vacuum to draw blow-by gases from the crankcase into the intake manifold for burning; Reduces HC, CO, and particulates
Aspirator valve (reed valve, gulp valve)
Uses variations in exhaust back pressure to allow fresh air into the exhaust manifold
Smog
Visible cloud of airborne pollutants, formed when pollutants combine with oxygen and nitrogen in the presence of sunlight
Fuel tank air dome
A hump formed in the top of the fuel tank that allows for fuel expansion without spillage, it provides about 10% air space to allow for fuel heating and expansion
EGR
Allows burned exhaust gases to enter the intake manifold to reduce combustion chamber temperature and reduce NOx emissions
Carbon Dioxide (CO2)
Byproduct of normal combustion that occurs when one carbon atom combines with two oxygen atoms, readings should be high, 8-15%
Air pollution
Caused by an excess amount of harmful chemicals in the atmosphere, emission control systems are used on cars and trucks to reduce those harmful chemicals
PCV valve
Controls flow through the system, it is located in the valve cover, or intake manifold; Varies flow for idle, cruise, acceleration, wide open throttle, and engine-off conditions
Oxides of Nitrogen (NoX)
Directly related to engine combustion temperatures, above approximately 2500 ºF (1370 ºC), nitrogen and oxygen combine
Air injection system
Forces fresh air into the exhaust ports or catalytic converter to reduce HC's and CO's; causes exhaust gas to continue to burn in the exhaust manifold or catalytic converter
Pollutants produced by internal combustion engines:
Hydrocarbons (HC) Carbon monoxide (CO) Oxides of nitrogen (NOx) Particulates
Pulse air system
Instead of a pump, this system uses natural pressure pulses in the exhaust system to operate the aspirator valves